1. Field of the Invention
The invention relates to pipeline systems particularly designed for withstanding elevated external compressive loads. More particularly, the invention relates to pipeline systems capable of withstanding increased stresses of the type resulting from use as oil or gas carriers in a submarine environment or as a liquefied natural gas carrier.
2. Description of the Prior Art
Cost, ease of installation, and resistance to internal and external stresses are important factors in the design of pipeline systems. Oil pipelines, gas pipelines, or especially liquefied natural gas (hereinafter referred to as "LNG") pipeline systems and submarine pipeline systems, are particularly vulnerable to stresses as described below, and each type of system must be designed to withstand these stresses.
LNG is generally transported by pipe at temperatures of -160.degree. C. Insulation material is conventionally used to maintain the low LNG temperature. Typically, the insulation material is surrounded by an outer pipe jacket made of material such as steel, plastic, or fiber reinforced plastic. The outer jacket serves to maintain the integrity of the insulation material, such as by protecting it from the ingress of water and water vapor, which would increase the thermal conductivity of the insulation. LNG pipelines are often placed in environments where they are subjected to elevated external compressive loads (for example, buried deep on the seafloor or underground), increasing the risk of jacket failure. Once the outer jacket has failed at one or more locations, the risk of water or water vapor penetrating the insulation is increased, resulting in an increase in the insulation's thermal conductivity and possible structural destruction of the insulation. The insulation's thermal conductivity can then be further increased by ice formation. Ingress of water or water vapor causes the outer jacket to be further weakened because the radical change in jacket temperature resulting from the increase in the insulation's thermal conductivity causes the lowering of jacket temperature and contraction of the jacket. Structural destruction of the insulation material may result. As the jacket is weakened, the likelihood of outer jacket failure, and the resulting ingress of water and water vapor, increases. Ultimately, ice builds up in the insulation, expanding to a volume where it destroys the outer jacket, leaving the LNG-carrying pipe exposed to the environment. An initial jacket failure can ultimately result in complete failure of the pipe itself, and finally, the escape of LNG into the environment.
For many applications, submarine pipeline systems must be able to withstand high external hydrostatic pressure since hydrostatic pressure increases at the rate of 1/2 pound per square inch per foot [approximately 10 kiloPascal (kPA) per meter]of water depth. For this reason, submarine pipeline systems conventionally are reinforced with an outer pipe jacket. Still, outer jackets are vulnerable to failure, especially when installed in sections. Small buckles which occur during installation can propagate along the jacket, causing collapse of the jacket, if the external hydrostatic pressure is greater than the initiation pressure (a function of the diameter to wall-thickness ratio and the jacket material grade). In order to handle elevated external hydrostatic pressures, conventional outer jackets are made of steel having large wall thickness. These solutions have disadvantages. Because conventional submarine pipelines typically have large diameters, increasing the density or thickness of the jacket material adds significantly to the cost and ease of installation of the system.
Jacketed pipelines for oil, gas and especially LNG often employ expansion bellows to withstand the thermal stress resulting from contrasting temperatures of the inner pipe and the outer jacket. The bellows, usually installed on the outer pipe jacket, do not totally prevent leakage, and destruction of the insulation material is still possible.
It is also conventional to use a denser insulation material in a submarine pipeline system than would be used for surface application, because more of the external compressive load can be supported. However, increasing the density of the insulation material results in an increase in thermal conductivity. So, while a large compressive load can be supported, the insulation will not be as effective.
The previously discussed problems associated with LNG pipelines are more severe for submarine LNG pipeline systems. The high external hydrostatic compressional load increases the risk that the outer jacket will leak and that the insulation will be flooded. The low temperature of the LNG carrying pipe aggravates the situation. The thermal conductivity of the insulation increases as water penetrates the outer jacket. As described above, the pipeline system can be destroyed as a result of ice formation inside the outer jacket.
It is an object of the present invention to provide an improved pipeline system capable of withstanding elevated external compressional loads, so that the risk of damage resulting from outer jacket failure is minimized.